As a fuel cell system mounted on a vehicle, a system having a plurality of DC-DC converters has been developed. For example, Patent Publication JP-A-2007-209161 discloses a fuel cell system having a first DC-DC converter disposed between a power storage device and an inverter and a second DC-DC converter disposed between a fuel cell and the inverter.
In the system, when a target motor output is larger than a predetermined threshold value, the first DC-DC converter is turned off, the second DC-DC converter is set into a directly connected state, and the output power of the fuel cell having high output is supplied to a motor in preference to the output power of the power storage device. On the other hand, when the target motor output is lower than the predetermined threshold value, the first DC-DC converter is operated to cause assistance power to be supplied from the power storage device, and the second DC-DC converter is put in an electrically directly connected state. With such configuration, the running performance of a vehicle has been prevented from being lowered, and effective power conversion has been allowed (Patent Literature 1).